Extremely Efficient Decomposition of Ammonia N to N2 Using ClO

Article Cite This: Environ. Sci. Technol. 2019, 53, 6945−6953

pubs.acs.org/est

Extremely Efficient Decomposition of Ammonia N to N2 Using ClO• from Reactions of HO• and HOCl Generated in Situ on a Novel Bifacial Photoelectroanode Yan Zhang,† Jinhua Li,† Jing Bai,*,† Linsen Li,† Shuai Chen,† Tingsheng Zhou,† Jiachen Wang,† Ligang Xia,‡ Qunjie Xu,‡,§ and Baoxue Zhou*,†,§,∥

Downloaded via LANCASTER UNIV on August 6, 2019 at 06:16:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China ‡ College of Environmental and Chemical Engineering, Shanghai University of Electric Power, 2588 Changyang Road, Shanghai 200090, People’s Republic of China § Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, People’s Republic of China ∥ Key Laboratory of Thin Film and Microfabrication Technology, Ministry of Education, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: The conversion of excess ammonia N into harmless N2 is a primary challenge for wastewater treatment. We present here a method to generate ClO• directionally for quick and efficient decomposition of NH4+ N to N2. ClO• was produced and enhanced by a bifacial anode, a front WO3 photoanode and a rear Sb−SnO2 anode, in which HO• generated on WO3 reacts with HClO generated on Sb− SnO2 to form ClO•. Results show that the ammonia decomposition rate of Sb−SnO2/WO3 is 4.4 times than that of WO3 and 3.3 times than that of Sb−SnO2, with achievement of the removal of NH4+ N on Sb−SnO2/WO3 and WO3 being 99.2 and 58.3% in 90 min, respectively. This enhancement is attributed to the high rate constant of ClO• with NH4+ N, which is 2.8 and 34.8 times than those of Cl• and HO•, respectively. The steady-state concentration of ClO• (2.5 × 10−13 M) is 102 times those of HO• and Cl•, and this is further confirmed by kinetic simulations. In combination with the Pd−Cu/NF cathode to form a denitrification exhaustion system, Sb−SnO2/WO3 shows excellent total nitrogen removal (98.4%), which is more effective than WO3 (47.1%) in 90 min. This study provides new insight on the directed ClO• generation and its application on ammonia wastewater treatment.

■

INTRODUCTION

resulting in slow and ineffective removal of total nitrogen from water.10−12 To overcome the above drawbacks, our group proposes a strategy for transforming NH4+ to N2 based on Cl• radical reactions.11 However, this method cannot achieve complete denitrification; 22% of NH4+ is still converted to NO3− and remains in the water. We then designed an exhaustive cycle system using Pd−Cu/NF as a cathode to convert inorganic nitrogen to N2.12 Unfortunately, the oxidation of NH4+ to N2 is relatively low, limited by the low concentration of Cl• in the system. As known, the generation of OH• and O2 are

The removal of ammonia nitrogen has been a primary challenge in wastewater treatment as a result of its characteristics of offensive smell, aquatic biota toxicity, conduction to eutrophication, and oxygen consumption in the nitrification process.1,2 Hence, focus on the enhancement of nitrogen removal has intensified, and a high standard is set for wastewater discharge. Various technologies have been developed to remove NH4+ N, including reverse osmosis,3 breakpoint chlorination,4 photocatalytic oxidation,5 and biological method.6 However, their drawbacks have been reported frequently. Biological treatment is greatly affected by the C/N ratio,7 and reverse osmosis processes require a secondary treatment.8 Breakpoint chlorination is impeded by the high cost and operational complexity.9 Photocatalytic/photoelectrochemical treatment only converts NH4+ N to NO3− N, © 2019 American Chemical Society

Received: Revised: Accepted: Published: 6945

February 15, 2019 April 29, 2019 May 22, 2019 May 22, 2019 DOI: 10.1021/acs.est.9b00959 Environ. Sci. Technol. 2019, 53, 6945−6953

Article

Environmental Science & Technology competitive processes,13,14 and OH• tends to convert chloride ions to ClOH• − and ClO•, instead of Cl• (eqs 1 and 2). HO• + Cl− → ClOH•−

1,4-dimethoxybenzene (DMOB) were obtained from Aladdin Industrial Corporation. Electrode Preparation. WO3 nanoplate arrays were fabricated on FTO via a hydrothermal reaction,22 and the detailed procedure is shown in the Supporting Information. To prepare the double-sided electrode, another side of FTO was protected by parafilm and all edges were sealed with adhesive tape. The Sb−SnO2 films were deposited on FTO by the spincoating method. A total of 1.402 g of SnCl4·5H2O, 0.406 g of SbCl3 and 1 g of PEG 6000 were dissolved in 20 mL of isopropanol and sonicated for 30 min to disperse evenly. The SnO2 precursor sol was spread on the FTO and then spincoated at 1000 rpm for 15 s, followed by drying at 350 °C for 5 min. This above cycle was repeated 6 times. Finally, the electrode was annealed at 500 °C for 2 h under atmospheric air (heating rate of 1 °C min−1). Figure S1 of the Supporting Information represents the schematic illustration for fabricating Sb−SnO2/WO3. Analytical Methods. X-ray photoelectron spectroscopy (XPS) was obtained using an Omicron EA125. Morphologies and elemental composition were characterized by Zeiss SUPRA55-VP field emission scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). The crystalline structure was examined by X-ray diffraction (XRD). Electron spin resonance (ESR) spectra were obtained with a Bruker electron paramagnetic resonance spectrometer (ESP 300E).26 The N,N-diethyl-p-phenylenediamine (DPD) method was used to determine the concentrations of free chlorine in the system.26 NO3− and NO2− were measured by ion chromatography (Dionex, Sunnyvale, CA, U.S.A.), and NH4+ was measured by Nessler reagent using an ultraviolet−visible (UV−vis) spectrophotometer at 420 nm. The photoelectrochemical performance of Sb−SnO2/WO3 was investigated by linear sweep voltammetry. The concentration of chlorate and perchlorate were identified by ion chromatography (Shimadzu HIC-10A, Japan) using an anion-exchange column (Shin-Pack IC-SA2). The total nitrogen (TN) removal was monitored by a multi N/C 3100 TOC/TN analyzer (Analytikjena, Germany). NB, BA, and DMOB were quantified by high-performance liquid chromatography (HPLC, LC20AT, Japan) coupled with a C18 column at 266, 227, and 230 nm, respectively. Degradation Experiment. The radical degradation of NH4+ N was performed in a single quartz reactor, and the WO3 side of the electrode was irradiated from a 150 W xenon lamp (Perfect, China). Sb−SnO2/WO3 was used as the anode, and a Pt sheet was used as the cathode. A total of 30 mg L−1 of NH4+ N solution was added to the cell. The initial pH was 5, and a constant potential of 1.7 V versus Ag/AgCl was applied. About 1 mL of solution was taken out at predetermined time intervals for the analysis of ammonia, nitrite, and nitrate concentrations. Determination of the Concentration of Radical Species. NB, BA, and DMOB were used as a probe to quantify the steady-state concentrations of HO•, Cl•, and ClO• in the WO3/Sb−SnO2 system (eqs 7−9). As known, NB only reacts with HO• (kHO•,NB = 3.9 × 109 M−1 s−1), while BA reacts with both HO• and Cl•19 (kHO•,BA = 5.9 × 109 M−1 s−1, and kCl•,BA = 1.8 × 1010 M−1 s−1). In addition, DMOB can react with HO•, Cl•, and ClO• (k HO•,DMOB = 7.0 × 109 M−1 s−1, kCl•,DMOB = 1.8 × 1010 M−1 s−1, and kClO•,DMOB = 2.1 × 109 M−1 s−1).

k = 4.30 × 109 M−1 s−1 (1)

ClOH•− → HO• + Cl−

k = 6.10 × 109 M−1 s−1 (2)

HO• + HOCl → ClO• + H 2O k = 2.0 × 109 M−1 s−1

(3)

HO• + OCl− → ClO• + OH− k = 8.8 × 109 M−1 s−1

(4)

In this study, we designed a method for the first time to generate ClO• directionally for rapid decomposition of NH4+ to N2, which is more efficient than the previous method of the Cl• reaction.11,12 Because ClO• has a high redox potential of 1.8 V,15 it reacts rapidly with electron-rich moieties.16,17 According to the literature, the concentration of ClO• was 102−103 times those of HO• and Cl• in the ultraviolet (UV)/ chlorine process18 and the reactivity can be higher than that of Cl• in some instances.19 Then, ClO• usually produced easily during the scavenging of •OH by free chlorine (HClO or ClO−),20 with a reaction rate of 109 M−1 s−1 (eqs 3 and 4).21 On the basis of this, we propose an idea to explore ClO• for rapid removal of NH4+ N. To realize this idea, ClO• was produced directionally and enhanced by a uniquely designed bifacial anode, a front WO3 photoanode and a rear Sb−SnO2 anode, in which HO• generated on WO3 reacts with HClO generated on Sb−SnO2 to form ClO•. In this hybrid anode, WO3 photoanode is selected for the generation of OH• because of visible light response, great hole mobility, and moderate hole diffusion length22 (eqs 5 and 6). The tin oxide doped with antimony (Sb−SnO2) is selected as a candidate for HOCl production as a result of excellent chlorine evolution activity23 and considerable electronic conductivity.24,25 WO3 + hν → h+ + e−

(5)

H 2O + h+ → HO• + H+

(6)

Thus, we report a novel double-sided structure for modifying WO3 with Sb−SnO2, which is used to generate ClO• in situ. The direct coating is avoided because Sb−SnO2 may fill the pores of WO3, thereby decreasing the photoelectrocatalytic performance. As-fabricated Sb−SnO2/WO3 shows synergetic effects on NH4+ N decomposition by coupling electrocatalysts and photoelectrocatalysts, and their performance is much superior to those of single WO3 and Sb−SnO2. With the construction of an exhaustive denitrification system with a Pd− Cu/NF cathode, Sb−SnO2/WO3 can completely convert total nitrogen into N2 in a short time. A combination of experimental and kinetic models is used to investigate ClO• generation mechanisms and model their steady-state concentrations in the system. This paper provides an economical and efficient method for sustainable wastewater treatment.

■

MATERIALS AND METHODS Materials. Fluorine-doped tin oxide (FTO) was purchased from Nippon Sheet Glass Co., Ltd. Analytical-grade Na2SO4, (NH4)2SO4, and NaCl were bought from Sinopharm Chemical Reagent Co., Ltd. Benzoic acid (BA), nitrobenzene (NB), and 6946

DOI: 10.1021/acs.est.9b00959 Environ. Sci. Technol. 2019, 53, 6945−6953

Article

Environmental Science & Technology

Figure 1. SEM images of (A and B) top views of the Sb−SnO2 layers, (C) cross-sectional views of the Sb−SnO2 layers, and (D) elemental mapping of the Sb−SnO2 layers.

Figure 2. (A) Survey spectra and (B−D) high-resolution spectra on Sn 3d, Sb 3d, and O 1s of the Sb−SnO2 electrode.

6947


Article


Figure 3. (A) NH4+ N removal and (B) plots of ln(C/Co) versus time for the NH4+ N degradation at different anodes. Conditions: pH 5, 50 mM NaCl, and 1.7 V versus Ag/AgCl.

KNB = k OH•,NB[HO•]

(7)

KBA = k OH•,BA[HO•] + k Cl•,BA[Cl•]

(8)

of 500 nm and tightly adhered to FTO. The formation of the Sb−SnO2 film was then studied by XRD in Figure S2 of the Supporting Information. The XRD pattern of the Sb−SnO2 film has three peaks at 2θ = 26.6°, 33.9°, and 53.0°, which were referenced to the (110), (101), and (211) planes of SnO2.30 In fact, no significant change was observed in XRD spectra after coating of Sb−SnO2, where only little peaks appear at 2θ = 19.1° and 29.2°. These peaks may be responsible for the presence of Sb2O4 peaks. To confirm the Sb doping, the compositional analysis for Sb−SnO2 was performed by EDS. The distribution of Sn, Sb, and O confirmed the uniform deposition of the Sb−SnO2 layer (Figure 1D). The surface morphology of the WO3 side was also investigated, and it appeared to be plate-like morphology (Figure S3A of the Supporting Information). From the cross-sectional SEM images in Figure S3B of the Supporting Information, the thickness of the WO3 film remained almost the same at 900 nm. In terms of the chemical bonding state of Sb−SnO2, the XPS measurement was recorded. The survey spectrum (Figure 2A) showed several strong peaks assigned to Sn 3d, O 1s, and Sb 3d, indicating that the layer was composed of SnO2 and Sb. In the Sn 3d region (Figure 2B), the spectrum had two peaks at 486.1 and 494.5 eV, and the interval is 8.4 eV, which agreed with the standard spectrum of SnO2. In Figure 2C, it exhibited two peaks at 539.2 and 540.1 eV, which correspond to Sb3+ and Sb5+, respectively. In Figure 2D, the spectra were difficult to assign as a result of the binding energies for O 1s overlap with Sb 3d.31 The O 1s spectra were deconvoluted into two independent peaks at 530.7 and 532.0 eV, which were associated with metallic oxides and hydroxides, respectively. Radical Degradation of NH4+ N on Sb−SnO2/WO3. Figure 3A showed the removal of NH4+ N by WO3/Sb−SnO2, WO3, and Sb−SnO2. The degradation processes were found to obey first-order kinetics. For WO3, about 12.5 mg L−1 NH4+ N was removed in 90 min and the rate constant was 0.009 min−1 (Figure 3B). For Sb−SnO2, 10.3 mg L−1 NH4+ N was degraded in 90 min and the rate constant was 0.012 min−1. When WO3/Sb−SnO2 was used, about 99.2% of NH4+ N was decomposed and the rate constant was 0.040 min−1, which was about 4.4 times that in the WO3 electrode, 3.3 times that in the Sb−SnO2 electrode, and even 1.9 times greater than the sum of individual WO3 and Sb−SnO2 electrodes.

KDMOB = k OH•,DMOF[HO•] + k Cl•,DMOF[Cl•] + k ClO•,DMOF[ClO•]

(9)

where KNB, KBA, and KDMOB are the rate constants for the degradation of NB, BA, and DMOB in WO3/Sb−SnO2, respectively. [HO•], [Cl•], and [ClO•] are the steady-state concentrations of HO•, Cl•, and ClO•, respectively. Determination of the Rate Constants between ClO• and NH4+. To exam the second-order rate constants of reaction between ClO• and NH4+, we use DMOB as the reference compound.27 DMOB can react with ClO• rapidly with a rate constant of 2.1 × 109 M−1 s−1. DMOB and NH4+ were added to the system at concentrations of 2 mM. NaHCO3 (100 mM) was used to quench HO• and Cl• in the system. + K ClO•,NH4+ ij [DMOB]o yz ji [NH4 ]o zyz zz z = lnjjj lnjjjj zz j [NH4 +] zz • [ ] DMOB K ClO ,DMOB k { { k

(10)

where KClO•,NH4+ and KClO•,DMOB represent the second-order rate constants of ClO• reacting with NH4+ and DMOB, respectively. Furthermore, the rate constants of OH• and Cl• reacting with NH4+ were also measured, and the details are shown in the Supporting Information. Kinetic Model Simulation. Kinetic modeling of the radical production during the experiment was performed using Kintecus 6.5 chemical kinetic modeling software equipped with a Bader−Deuflhard integrator.28 The model established in this study contained 24 reactions (Table S1 of the Supporting Information).

■

RESULTS AND DISCUSSION Characterization of the Bifacial Anode. The morphologies of Sb−SnO2 were characterized by SEM. Morphologies of the Sb−SnO2 layer prepared by spin coating were dense and smooth (Figure 1A). In Figure 1B, a uniform and less “crackmud” structure was presented, which could favor the improvement of stability.29 From cross-sectional SEM in Figure 1C, Sb−SnO2 was a sponge-type film with a thickness 6948


Article


Figure 4. (A) Variation of the free chlorine concentration during the reaction, (B) ESR spectra in the Sb−SnO2/WO3 and WO3 systems, (C) NH4+ N degradation in Sb−SnO2/WO3 with the addition of different scavengers, (D) NH4+ N degradation rate constant in Sb−SnO2/WO3, Sb− SnO2, and WO3 with the addition of different scavengers.

current generated on the bifacial electrodes is lower than that of Sb−SnO2 under the same potential as a result of the positive shift of chorine evolution potential. Under irradiation, Sb− SnO2/WO3 became similar to that of WO3. In Figure S5B of the Supporting Information, Sb−SnO2/WO3 has the largest current, which is consistent with LSV results. This indicates that Sb−SnO2/WO3 possesses both conductive and semiconductive characteristics. Reactive Chlorine Generation. As known, the electrochemical NH4+ N oxidation process commences with the oxidation of Cl− to HClO.33,34 We therefore investigated the variations of free chlorine in Sb−SnO2, WO3, and Sb−SnO2/ WO3. As shown in Figure 4A, free chlorine gradually increased to 254 mg L−1 in Sb−SnO2/WO3, which was lower than the sum of individual WO3 and Sb−SnO2. It indicated that free chlorine was not primarily responsible for degradation of NH4+ N in Sb−SnO2/WO3. Subsequently, the ESR technique was performed using DMPO as a spin-trap reagent to identify the oxidative radicals. The signals of DMPO−OH • were detected,34 indicating the generation of OH• (Figure 4B). The signal of DMPO−ClO• was also observed,35 and other peaks of low intensity may correspond to Cl•.36,37 The signals intensities of OH• and ClO• in Sb−SnO2/WO3 were stronger than those in WO3. Therefore, radical-dominated oxidation took responsibility in Sb−SnO2/WO3. To clarify the roles of • OH on the generation •ClO in this system, we compared the NH4+ N degradation by HClO in the absence or presence of the WO3 photoanode and found that the degradation rates

Subsequently, we use the synergy index to study the synergy effects of WO3/Sb−SnO2 on the degradation of NH4+ N according to the previous studies.32 S = 100% ×

K Sb−SnO2 /WO3 − K Sb−SnO2 − K WO3 K Sb−SnO2 /WO3

(11)

S was calculated as 47.5%, which indicated that there was a significant synergistic effect between photoelectrocatalysis and electrocatalysis. This synergistic effect may be due to enhanced chorine evolution. We evaluated the electrocatalytic activities of WO3/Sb−SnO2 in two different electrolyte solutions: a saturated NaCl solution for chorine evolution reaction (CER) and a 0.2 M NaH2PO4 solution for oxygen evolution reaction (OER). As shown in Figure S4 of the Supporting Information, the currents for WO3 in NaCl and NaH2PO4 solutions were weaker than those for WO 3/Sb−SnO 2 . WO 3/Sb−SnO 2 displayed a higher activity in NaCl than in NaH2PO4; therefore, WO3/Sb−SnO2 was more efficient for CER than for OER. We monitored the linear sweep voltammetry (LSV) of Sb− SnO2, WO3, and Sb−SnO2/WO3 in 50 mM NaCl under dark and irradiation conditions, separately. In Figure S5A of the Supporting Information, Sb−SnO2 showed an abrupt increase of current at 1.25 V, which was due to the reactive chlorine species (RCS) generation. In contrast, WO3 did not generate current at 1.25 V. A slight increase was observed with potential above 1.6 V. The behavior of Sb−SnO2/WO3 is unique. It showed Sb−SnO2-like behavior in the absence of light. The 6949


Article

Environmental Science & Technology significantly increased when WO3 was used, revealing that HClO may be converted to more active ClO• (Figure S6A of the Supporting Information). Then, the ESR test was carried out to confirm the •ClO generation in this WO3/HClO process (Figure S6B of the Supporting Information). To confirm the contributions of radicals, experiments are carried out with the addition of different kinds of scavengers. NB can only react with OH• rapidly, while TBA can scavenge HO•, Cl•, and ClO•.38 In addition, HCO3− is used to scavenge HO• and Cl•, while its reaction with ClO• was negligible.27 The rate constants of scavengers reacting with different radicals are listed in Table S2 of the Supporting Information. In Figure 4C, TBA could completely inhibit NH4+ N degradation, indicating that the radical reaction played a dominant role. In contrast, only 13% of degradation was inhibited in the presence of NB, implying that OH• is not the major radical for the direct conversion of NH4+ N to N2. We also found that NH4+ N degradation was not completely inhibited with the addition of NaHCO3 , suggesting that ClO • was the predominant oxidant. As displayed in Figure S7 of the Supporting Information, the contribution of CO3• − is excluded because NH4+ N was hardly degraded in 50 mM NaHCO3 electrolyte. The scavenging experiments were also carried out in WO3 and Sb−SnO2 electrodes (Figure 4D). Different from Sb−SnO2/WO3, NH4+ N degradation rates significantly decreased with the addition of NaHCO3, revealing that ClO• is not involved in WO3 and Sb−SnO2 electrodes. To identify the concentrations of HO•, Cl•, and ClO• in the Sb−SnO2/WO3 system, different probes are used. As shown in Figure S8 of the Supporting Information, KNB, KBA, and KDMOB are 0.0054, 0.0094, and 0.038 min−1, respectively, According to eqs 7−9, [HO•], [Cl•], and [ClO•] are then examined to be 4.03 × 10−15, 4.14 × 10−16, and 2.53 × 10−13 M, respectively. It shows that the concentration of ClO• is 102−103 times higher than those of HO• and Cl•, which means that ClO• is the dominant radical. The steady-state concentrations of HO•, Cl•, and ClO• were then estimated using the kinetic model. Even though the generations of OH• and Cl• had initial radical formation steps, the modeling results also showed that ClO• was the dominant radical (Figure S9 of the Supporting Information), which was consistent with the experimental results. To better explain the reactivity of ClO• with NH4+, we thus investigate the rate constants in the Sb−SnO2/WO3 system (Figure S10 of the Supporting Information). The second-order rate constants of ClO• with NH4+ are calculated to be 3.1 × 109 M−1 s−1, which is 2.8 times that of Cl• (1.1 × 109 M−1 s−1) and 34.8 times that of HO• (8.9 × 107 M−1 s−1). It demonstrates that ClO• plays important roles in the transformations of NH4+ N to N2. ClO• Generation Mechanisms of the Bifacial Electrode. On the basis of the experimental and modeling results, a reasonable schematic mechanism for treating NH4+ N was illustrated (Figure 5). Under irradiation, the photogenerated hole yield at the WO3 surface can oxidize water to OH• and chlorine ion to Cl•, respectively. On the Sb−SnO2 side, chloride oxidation occurs efficiently in a low potential, creating free chlorine, such as HClO and ClO− (eqs 12−15). Free chlorine would rapidly quench OH• and Cl• generated on the WO3 surface, forming a large number of ClO• (eqs 16−18). The reaction of NH4+ N with •ClO may proceed via the addition of Cl to H abstraction.39 The •ClO radical will take H on the NH4+ molecule to form the N-centered radicals (•NH2

Figure 5. ClO• generation mechanisms on the Sb−SnO2/WO3 bifacial electrode for NH4+ N degradation.

and •NHCl), and N-centered radicals are unstable and can gradually convert to N240 (eqs 19−26). The experimental results indicate that N2 is the primary product during NH4+ N oxidation. NH4+ degradation may also generate some nitrate via a series of radical reactions (eqs 27−29). Cl− + h+ → Cl• −

2Cl → Cl 2 + 2e

(12) −

(13)

Cl 2 + H 2O → HClO + HCl −

−

(14)

−

Cl + 2OH → OCl + H 2O + 2e

−

(15)

•

Cl + HOCl → ClO• + H+ + Cl−

(16)

•

OH + HOCl → ClO• + H 2O

(17)

•

OH + OCl− → ClO• + OH− +

•

(18)

•

NH4 + ClO → NH 2 + HClO + H •

NH 2 + HClO → NH 2Cl + OH •

+

•

(20)

•

NH 2Cl + ClO → NHCl + HClO •

NHCl + HClO → NHCl 2 + OH

(19)

(21)

•

+

(22) −

NHCl 2 + H 2O → NOH + 2H + 2Cl

(23)

NH 2Cl + NOH → N2 + H+ + Cl− + H 2O

(24)

NHCl 2 + NOH → N2 + 2H+ + 2Cl−

(25)

NHCl 2 + NH 2Cl → N2 + 3H+ + 3Cl−

(26)

•

NH 2 + •OH → NH 2OH

(27)

NH 2OH + •OH → NO2− → NO3−

(28)

4ClO− + NH4 + → NO3− + H 2O + Cl− + 2H+

(29)

Effect of the Anodic Potential. Figure S11A of the Supporting Information showed the effect of potential on the degradation of NH4+ N. When the potential increased from 1.1 to 2.0 V, the removal efficiency of ammonia N increased from 51.3 to 99.8%. This result indicated that a higher potential was beneficial for ammonia N degradation. When potential was low, the generation of HClO on Sb−SnO2 decreased and it only inhibited the recombination of the electron and hole, 6950


Article


Figure 6. Effects of the chloride concentration on (A) NH4+ N removal and (B) NO3− N generation. Conditions: pH 5, 1.7 V versus Ag/AgCl, and 30 mg L−1 NH4+ N.

we should control the pH to inhibit the NO3− N yield, and the optimum pH is 5. The effects of Sb−SnO2 dosages were also examined. In Figure S13 of the Supporting Information, an inadequate dose of Sb−SnO2 induced cracks and faults on the electrode surfaces. For excessive Sb−SnO2 dosages, polymerized particles appeared. The electrochemical performance was then tested. In Figure S14A of the Supporting Information, a six-layer Sb−SnO2 film had the highest current, suggesting the more efficient generation of RCS. The degradation of NH4+ N also verified that the optimal activity was obtained with the Sb−SnO2 coating of six layers (Figure S14B of the Supporting Information). The Sb−SnO2/WO3 system was applied to treat ammonia wastewater with a high concentration. Although the efficiencies decreased as the initial concentration increased, the absolute amount of NH4+ N removal increased. It showed that 100 mg L−1 NH4+ N could be removed (88%) in this system after 120 min of treatment (Figure S15 of the Supporting Information). The generation of byproducts, such as chlorate and perchlorate, were investigated. In this system, chlorate was the main byproduct, with the concentration of perchlorate below the detection limit. In Figure S16 of the Supporting Information, the concentration of ClO3− increased with the reaction time, and the final concentrations was 0.32 mM, which was less than 1% of the initial Cl− concentration. It indicated that the byproduct generation in our system was suppressed. Stability of the Electrode. The stability of the WO3/Sb− SnO2 anode is important for practical application. In Figure S17 of the Supporting Information, the removal of NH4+ N is nearly maintained at the level of the fresh sample after 5 consecutive runs. Furthermore, the SEM patterns of the used anode are found to be similar to that recorded before reaction (Figure S18 of the Supporting Information), which has potential for long-term application. The current efficiency and energy consumption were also evaluated. As shown in Figure S19 of the Supporting Information, the current efficiency first increased with the increase in potential and then decreased, with the maximum value (64.3%) achieved at 1.7 V. In addition, this system consumed the lowest energy (16.4 kW h kg−1 of N) at 1.7 V, confirming that it is a low-cost system for ammonia wastewater treatment. The Sb−SnO2/WO3 system was used to treat real wastewater after filtration, and the main characteristics were

which would reduce the synergy of the bifacial electrode. As shown in Figure S11B of the Supporting Information, excessive potential would convert ammonia into more nitrate, which may be due to direct oxidation of NH4+ by surface OH• on the anode.41 With the two aspects taken into consideration, the potential of 1.7 V was chosen for the subsequent experiments. Effect of the Chloride Ion Concentration. To study the effect of the chlorine ion on NH4+ N degradation, different chlorine concentrations were used. In Figure 6A, when the chlorine concentration was 0 mM, the NH4+ N concentration declined from 30 to 19.1 mg L−1 in 90 min. The degradation may derive from photoelectrocatalysis of the WO3 anode. The NH4+ N removal increased sharply from 36.3 to 99.7% with increasing chlorine ions from 0 to 75 mM. The results suggested that the presence of chloride ions was beneficial for the degradation of NH4+ N in the WO3/Sb−SnO2 system. In this study, the degradation of NH4+ N by WO3/Sb−SnO2 may proceed through the formation of ClO•. In addition to accelerating the degradation of NH4+ N, the chloride ion also affects degradation products. In this study, N2 and NO3− were the main products, while NO2− was not detected throughout the reaction. In Figure 6B, the concentration of NO3− N increased to 7.87 mg L−1 without the chloride ion, which means that most NH4+ was converted to NO3−. The nitrate concentration significantly decreased with an increasing chlorine ion concentration. However, nitrate production would increase with a further increase of the chlorine concentration to 75 mM. As known, it is reasonable to convert NH4+ to N2 directly without other byproducts, such as NO3−. Therefore, 50 mM chloride ion was the most active catalyst for NH4+ oxidation to N2. Effect of the pH Value. Figure S12A of the Supporting Information shows the degradation of NH4+ N at different pH values. It shows that NH4+ N degradation is enhanced with decreasing the pH. As for acidic conditions, the reduction of H+ to H2 would increase, which then improved the electron transfer from the anode to the cathode. When solution is strongly alkaline, NH4+ N removal is significantly retarded. WO3 may be damaged by corrosion in strong alkali conditions, which reduced the oxidation ability of WO3/Sb−SnO2. In fact, the pH can remarkably influence the products. Although NH4+ N removal increases in lower pH values, we find that NH4+ N is easily converted to nitrate. Along with decreasing the pH from 5 to 3, the yield of NO3− increased from 2.7 to 3.5 mg L−1 (Figure S12B of the Supporting Information). Therefore, 6951


Article


Electronic Materials and Devices (AEMD) of Shanghai Jiao Tong University (SJTU) for support.

shown in Table S3 of the Supporting Information. The deficiency of chloride ions was made up by adding NaCl. As displayed in Figure S20A of the Supporting Information, about 43.4 mg L−1 NH4+ N was removed after 90 min of treatment and the TN removal was 81.2%. The effect of anions in water was also evaluated on the degradation of NH4+ N, and the results are shown in Figure S20B of the Supporting Information. We found that NH4+ N could be efficiently removed in the presence of different anions (bicarbonate, nitrate, and phosphate), although its efficiency slightly decreased, indicating that the main water anions do not significantly affect the performance of this system. Sb−SnO2/WO3 has been shown to rapidly convert NH4+ N to N2 via ClO•. However, about 14% of NH4+ is converted to NO3− in 90 min. To complete remove of nitrogen from water, the Pt electrode is replaced with a Pd−Cu-modified Ni foam cathode, in which NO3− can be reduced to N2 (the mechanism shown in Figure S21 of the Supporting Information). In Figure S22 of the Supporting Information, we found that this system can effectively treat NH4+ wastewater and the TN removal efficiency was increased to 98.4% within 90 min, which is 2.1 times higher than that of WO3 (47.1%).

■

■

(1) Xu, W.; Du, D.; Lan, R.; Humphreys, J.; Miller, D. N.; Walker, M.; Wu, Z.; Irvine, J. T. S.; Tao, S. Electrodeposited NiCu bimetal on carbon paper as stable non-noble anode for efficient electrooxidation of ammonia. Appl. Catal., B 2018, 237, 1101−1109. (2) Zhang, X.; Wu, Y.; Liu, X.; Reis, S.; Jin, J.; Dragosits, U.; Van Damme, M.; Clarisse, L.; Whitburn, S.; Coheur, P.-F.; Gu, B. Ammonia Emissions May Be Substantially Underestimated in China. Environ. Sci. Technol. 2017, 51 (21), 12089−12096. (3) Masse, L.; Masse, D.; Pellerin, Y. The effect of pH on the separation of manure nutrients with reverse osmosis membranes. J. Membr. Sci. 2008, 325 (2), 914−919. (4) Pressley, T. A.; Bishop, D. F.; Roan, S. G. Ammonia-nitrogen removal by breakpoint chlorination. Environ. Sci. Technol. 1972, 6 (7), 622−628. (5) Wang, H.; Su, Y.; Zhao, H.; Yu, H.; Chen, S.; Zhang, Y.; Quan, X. Photocatalytic oxidation of aqueous ammonia using atomic single layer graphitic-C3N4. Environ. Sci. Technol. 2014, 48 (20), 11984−90. (6) Liu, H.; Zhu, L.; Tian, X.; Yin, Y. Seasonal variation of bacterial community in biological aerated filter for ammonia removal in drinking water treatment. Water Res. 2017, 123, 668−677. (7) Zhan, G.; Zhang, L.; Li, D.; Su, W.; Tao, Y.; Qian, J. Autotrophic nitrogen removal from ammonium at low applied voltage in a singlecompartment microbial electrolysis cell. Bioresour. Technol. 2012, 116, 271−7. (8) Bodalo, A.; Gomez, J.-L.; Gomez, E.; Leon, G.; Tejera, M. Ammonium removal from aqueous solutions by reverse osmosis using cellulose acetate membranes. Desalination 2005, 184 (1−3), 149− 155. (9) Jafvert, C. T.; Valentine, R. L. Reaction scheme for the chlorination of ammoniacal water. Environ. Sci. Technol. 1992, 26 (3), 577−586. (10) Xiao, S.; Wan, D.; Zhang, K.; Qu, H.; Peng, J. Enhanced photoelectrocatalytic degradation of ammonia by in situ photoelectrogenerated active chlorine on TiO2 nanotube electrodes. J. Environ. Sci. (Beijing, China) 2016, 50, 103−108. (11) Zhang, X.; Li, W.; Ren, P.; Wang, X. Chlorine/UV induced photochemical degradation of total ammonia nitrogen (TAN) and process optimization. RSC Adv. 2015, 5 (78), 63793−63799. (12) Wang, D.; Li, Y.; Li Puma, G.; Wang, C.; Wang, P.; Zhang, W.; Wang, Q. Dye-sensitized photoelectrochemical cell on plasmonic Ag/ AgCl@chiral TiO2 nanofibers for treatment of urban wastewater effluents, with simultaneous production of hydrogen and electricity. Appl. Catal., B 2015, 168−169, 25−32. (13) Menzel, N.; Ortel, E.; Mette, K.; Kraehnert, R.; Strasser, P. Dimensionally Stable Ru/Ir/TiO2-Anodes with Tailored Mesoporosity for Efficient Electrochemical Chlorine Evolution. ACS Catal. 2013, 3 (6), 1324−1333. (14) Exner, K. S.; Anton, J.; Jacob, T.; Over, H. Controlling selectivity in the chlorine evolution reaction over RuO2-based catalysts. Angew. Chem., Int. Ed. 2014, 53 (41), 11032−5. (15) Alfassi, Z. B.; Huie, R. E.; Mosseri, S.; Neta, P. Kinetics of oneelectron oxidation by the ClO radical. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry 1988, 32 (1), 85−88. (16) Kong, X.; Wu, Z.; Ren, Z.; Guo, K.; Hou, S.; Hua, Z.; Li, X.; Fang, J. Degradation of lipid regulators by the UV/chlorine process: Radical mechanisms, chlorine oxide radical ClO•-mediated transformation pathways and toxicity changes. Water Res. 2018, 137, 242− 250. (17) Wu, Z.; Fang, J.; Xiang, Y.; Shang, C.; Li, X.; Meng, F.; Yang, X. Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways. Water Res. 2016, 104, 272−282. (18) Sun, P.; Lee, W.-N.; Zhang, R.; Huang, C.-H. Degradation of DEET and Caffeine under UV/Chlorine and Simulated Sunlight/

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b00959.

■

REFERENCES

Description of the preparation of the WO3 photoanode and analytical methods of NB, BA, and DMOB, SEM, XRD, and LSV measurements of the Sb−SnO2/WO3 photoanode, model simulation results, second-order rate constants of NH4+ reacting with ClO•, Cl•, and OH•, degradation of NH4+ N by HClO in the presence of WO3, removal of NH4+ N under different pH, applied potential, Sb−SnO 2 layers, and NH 4 + N initial concentration, time profiles of ClO3− ions, current efficiency and energy consumption, stability test of Sb− SnO2/WO3, performance of Sb−SnO2/WO3 for degrading real wastewater, total nitrogen removal mechanism and performance in the exhaustive denitrification system, details of the kinetic model, rate constants of scavengers reacting with different radicals, and actual wastewater composition (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-21-54747351. E-mail: bai_jing@sjtu. edu.cn. *Telephone/Fax: +86-21-54747351. E-mail: zhoubaoxue@ sjtu.edu.cn. ORCID

Baoxue Zhou: 0000-0001-6957-190X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (21875139 and 21776177), Shanghai International Science and Technology Cooperation Fund Project (No. 18520744900) and the Center for Advanced 6952


Article


removal: Mechanism and kinetics. J. Hazard. Mater. 2019, 368, 234− 242. (36) Zhang, Y.; Li, J.; Bai, J.; Shen, Z.; Li, L.; Xia, L.; Chen, S.; Zhou, B. Exhaustive Conversion of Inorganic Nitrogen to Nitrogen Gas Based on a Photoelectro-Chlorine Cycle Reaction and a Highly Selective Nitrogen Gas Generation Cathode. Environ. Sci. Technol. 2018, 52 (3), 1413−1420. (37) Li, T.; Jiang, Y.; An, X.; Liu, H.; Hu, C.; Qu, J. Transformation of humic acid and halogenated byproduct formation in UV-chlorine processes. Water Res. 2016, 102, 421−427. (38) Cheng, X.; Liu, H.; Chen, Q.; Li, J.; Wang, P. Preparation and characterization of palladium nano-crystallite decorated TiO2 nanotubes photoelectrode and its enhanced photocatalytic efficiency for degradation of diclofenac. J. Hazard. Mater. 2013, 254−255, 141−8. (39) Wang, L.; Arey, J.; Atkinson, R. Reactions of chlorine atoms with a series of aromatic hydrocarbons. Environ. Sci. Technol. 2005, 39 (14), 5302−5310. (40) De Laat, J.; Boudiaf, N.; Dossier-Berne, F. Effect of dissolved oxygen on the photodecomposition of monochloramine and dichloramine in aqueous solution by UV irradiation at 253.7 nm. Water Res. 2010, 44 (10), 3261−3269. (41) Song, S.; Liu, Z.; He, Z.; Zhang, A.; Chen, J.; Yang, Y.; Xu, X. Impacts of Morphology and Crystallite Phases of Titanium Oxide on the Catalytic Ozonation of Phenol. Environ. Sci. Technol. 2010, 44 (10), 3913−8.

Chlorine Conditions. Environ. Sci. Technol. 2016, 50 (24), 13265− 13273. (19) Fang, J.; Fu, Y.; Shang, C. The roles of reactive species in micropollutant degradation in the UV/free chlorine system. Environ. Sci. Technol. 2014, 48 (3), 1859−68. (20) Wu, Z.; Guo, K.; Fang, J.; Yang, X.; Xiao, H.; Hou, S.; Kong, X.; Shang, C.; Yang, X.; Meng, F.; Chen, L. Factors affecting the roles of reactive species in the degradation of micropollutants by the UV/ chlorine process. Water Res. 2017, 126, 351−360. (21) Hua, Z.; Guo, K.; Kong, X.; Lin, S.; Wu, Z.; Wang, L.; Huang, H.; Fang, J. PPCP degradation and DBP formation in the solar/free chlorine system: Effects of pH and dissolved oxygen. Water Res. 2019, 150, 77−85. (22) Kalanur, S. S.; Hwang, Y. J.; Chae, S. Y.; Joo, O. S. Facile growth of aligned WO3 nanorods on FTO substrate for enhanced photoanodic water oxidation activity. J. Mater. Chem. A 2013, 1 (10), 3479−3488. (23) Yang, S. Y.; Kim, D.; Park, H. Shift of the reactive species in the Sb−SnO2-electrocatalyzed inactivation of e. coli and degradation of phenol: Effects of nickel doping and electrolytes. Environ. Sci. Technol. 2014, 48 (5), 2877−84. (24) Cho, K.; Qu, Y.; Kwon, D.; Zhang, H.; Cid, C. A.; Aryanfar, A.; Hoffmann, M. R. Effects of anodic potential and chloride ion on overall reactivity in electrochemical reactors designed for solarpowered wastewater treatment. Environ. Sci. Technol. 2014, 48 (4), 2377−84. (25) Sun, N.; Qu, Y.; Chen, S.; Yan, R.; Humayun, M.; Liu, Y.; Bai, L.; Jing, L.; Fu, H. Efficient photodecomposition of 2, 4dichlorophenol on recyclable phase-mixed hierarchically structured Bi2O3 coupled with phosphate-bridged nano-SnO2. Environ. Sci.: Nano 2017, 4 (5), 1147−1154. (26) Zhang, Y.; Li, J.; Bai, J.; Li, X.; Shen, Z.; Xia, L.; Chen, S.; Xu, Q.; Zhou, B. Total organic carbon and total nitrogen removal and simultaneous electricity generation for nitrogen-containing wastewater based on the catalytic reactions of hydroxyl and chlorine radicals. Appl. Catal., B 2018, 238, 168−176. (27) Guo, K.; Wu, Z.; Shang, C.; Yao, B.; Hou, S.; Yang, X.; Song, W.; Fang, J. Radical Chemistry and Structural Relationships of PPCP Degradation by UV/Chlorine Treatment in Simulated Drinking Water. Environ. Sci. Technol. 2017, 51 (18), 10431−10439. (28) Yang, Y.; Shin, J.; Jasper, J. T.; Hoffmann, M. R. Multilayer Heterojunction Anodes for Saline Wastewater Treatment: Design Strategies and Reactive Species Generation Mechanisms. Environ. Sci. Technol. 2016, 50 (16), 8780−7. (29) Li, X.; Shao, D.; Xu, H.; Lv, W.; Yan, W. Fabrication of a stable Ti/TiOxHy/Sb−SnO2 anode for aniline degradation in different electrolytes. Chem. Eng. J. 2016, 285, 1−10. (30) Tang, B.; Shi, H.; Fan, Z.; Zhao, G. Preferential electrocatalytic degradation of 2,4-dichlorophenoxyacetic acid on molecular imprinted mesoporous SnO2 surface. Chem. Eng. J. 2018, 334, 882−890. (31) Chen, Y.; Tu, Y.; Zhang, Y.; Lu, J.; Fang, B. Fabrication and enhanced electrocatalytic activity of TiO2 nanotubes based threedimensionally macroporous SnO2 with mesoporous walls. Chem. Eng. J. 2017, 311, 100−110. (32) Frontistis, Z.; Daskalaki, V. M.; Hapeshi, E.; Drosou, C.; FattaKassinos, D.; Xekoukoulotakis, N. P.; Mantzavinos, D. Photocatalytic (UV-A/TiO2) degradation of 17α-ethynylestradiol in environmental matrices: Experimental studies and artificial neural network modeling. J. Photochem. Photobiol., A 2012, 240, 33−41. (33) Zhang, C.; He, D.; Ma, J.; Waite, T. D. Active chlorine mediated ammonia oxidation revisited: Reaction mechanism, kinetic modelling and implications. Water Res. 2018, 145, 220−230. (34) Xiao, S.; Qu, J.; Zhao, X.; Liu, H.; Wan, D. Electrochemical process combined with UV light irradiation for synergistic degradation of ammonia in chloride-containing solutions. Water Res. 2009, 43 (5), 1432−40. (35) Hao, R.; Mao, X.; Wang, Z.; Zhao, Y.; Wang, T.; Sun, Z.; Yuan, B.; Li, Y. A novel method of ultraviolet/NaClO2-NH4OH for NO 6953


Extremely Efficient Decomposition of Ammonia N to N2 Using ClO

Recommend Documents